1. Field of the Invention
The present invention relates to a mobile radio communication apparatus that performs a control process to seize a base station. More particularly, it relates to a mobile radio communication apparatus that can seize one of the base stations in the descending order of priority assigned to the base stations.
2. Description of the Related Art
Every mobile radio communication system comprises base stations, mobile radio communication apparatuses, each mobile apparatus connected to one of the base stations over a radio channel. In a mobile radio communication system, any mobile radio communication apparatus starts a control process when its power switch is turned on. Upon finishing the control process, the apparatus seizes the base station and takes an idle state. In the idle state, the apparatus receives a call from the base station seized. While remaining in the idle state, the apparatus can transmit the user's calls origination to the base station.
Generally, the areas covered by base stations overlap one another, and the base stations offer same service at different charges. The mobile radio communication apparatus stores information representing a priority corresponding to each base station. Priority is allocated to each base station in accordance with the charge at which they provide the service. More precisely, the lower the charge, the higher the priority. Thus, the apparatus tries to seize, first the base station whose charge is the lowest, next the base station whose charge is the second lowest, and so forth.
The apparatus is designed on the assumption that the user turns off the apparatus in the idle state and turn it on again, in most cases in the same place. Thus, when the apparatus is turned on again, it immediately seizes the same base station it seized last, saving time for the control process. Thus, the mobile radio communication apparatus can save the battery power, in the following specific method.
The apparatus incorporates an MRU (Most Recent Used) memory. The MRU memory stores the data representing a system ID number and a frequency that corresponds to the system ID number (SID) of the base station seized when the apparatus was turned off last. When the apparatus is turned on, the MRU memory is searched for the SID based on the stored frequency and the stored SID. If the SID is detected, the apparatus seizes the base station having this SID and takes an idle state. Thus, the apparatus need not examine the priority allocated to the base station which is stored in the apparatus in order to seize the base station again. This helps to shorten the time the apparatus may spend to take an idle state.
However, this method is disadvantageous in the following respect.
Assume that four base stations A1, A2, A3 and B are operating in the mobile communication system in which the mobile radio communication apparatus is used, as is illustrated in FIG. 11. The base stations A1, A2, A3 and B cover four service areas, which overlap one another as shown in FIG. 11. Suppose an order of the priority that the user of the apparatus wants to seize is the base station A1, base station A2, base station A3 and base station B. That is, the base station B which covers the area indicated by a broken circle has the lowest priority.
The apparatus may exist in a shaded part of the service area of the base station B, which does not overlap the service areas of the other base stations A1 to A3. In this case, the apparatus cannot seize the base stations A1, A2 or A3 having priority higher than that of the base station B. This is because no radio signals broadcast from the base stations A1 to A3 reach the apparatus. Thus, the apparatus seizes the base station B and takes the idle state. When the apparatus goes into the idle state, the SID44 assigned to the base station B is stored into the MRU memory. Thereafter, the user may turned off the apparatus and move to a position X, bringing the apparatus with the user. The position X lies in both the service area of the base station B and that of the base station A1. Hence, the apparatus can now seize the base station A1 that has higher priority than the base station B, as the user desires. Nonetheless, the conventional apparatus seizes the base station B based on SID44 stored in the MRU memory when the user turns on the apparatus.
As described above, when the conventional mobile radio communication apparatus is turned off, the SID number of the base station had been stored into the MRU memory. When the apparatus is turned on thereafter, it refers to the stored SID number, even if it has been moved to a position in both the service area of that base station and the service area of any other base station having high priority and the position being therefore more desirable for the user. Obviously, this is inconvenient to the user who wishes to receive service at the lowest possible charge, for example.